(1) Field of the Invention
The present invention relates to an electrically conductive thermoplastic resin composition. More particularly, the present invention relates to an electrically conductive thermoplastic resin composition containing a relatively small amount of electrically conductive fillers evenly dispersed in a thermoplastic resin matrix, and having a satisfactory electrical conductivity.
(2) Description of Prior Art
It is well known that the electrical conductivity of a thermoplastic resin can be increased by mixing an electrically conductive filler, for example, graphite powder, carbon black powder, carbon fibers, or metallic fibers, into a thermoplastic resin. In the known electrically conductive fillers, graphite powder, carbon black powder, and carbon fibers have a relatively low specific gravity and exhibit a lower electrical conductivity than the metallic fibers. Therefore, to obtain a satisfactory electrical conductivity by using one of the above-mentioned carbonous fillers, it is necessary to mix a relatively large amount of the carbonous filler into the thermoplastic resin. This necessity causes the resultant thermoplastic resin composition to exhibit a decreased moldability and degraded mechanical properties, for example, poor elongation and reduced impact strength.
The metallic fibers, for instance, brass fibers, are highly effective for imparting a high electrical conductivity to the thermoplastic resin, even when mixed in a relatively small amount compared to the carbonous fillers. However, the metallic fibers have a high specific gravity and, therefore, the resin composition containing the metallic fibers exhibits an undesirable high specific gravity.
Japanese Unexamined Patent Publication (Kokai) No. 56-72049 (1981) discloses an electrically conductive resin composition comprising a thermoplastic resin matrix and a filler consisting of a carbon powder, such as graphite powder or carbon black powder, and metallic fibers, dispersed in the matrix. In this type of electrically conductive resin composition, the electrically conductive filler must be mixed in a large amount of 20% to 30% by volume into the matrix to provide a satisfactory electrical conductivity of the resultant composition. Especially, when the amount of the metal fibers is limited to 3% to 10% by volume, the amount of the carbon powder must be more than 10% by volume, for example, from 10% to 20% by volume. This large content of the electrically conductive filler results in a poor moldability and in undesirable mechanical properties of the resultant resin composition.